There exists today a need to provide semiconductor packages which have a very great number of leads but which are also of a very small overall size. One component of a semiconductor package is a "lead frame", which is a conductive metal part having a plurality of leads for connecting to the semiconductor chip, such that the leads will extend from the package for connection to a circuit board when the semiconductor device is eventually incorporated into an electronic device. The lead frame will also have one or more joining strips for combining the leads into a unitary structure during the semiconductor manufacturing process. After the leads are affixed to the semiconductor chip, the joining strips are removed from the leads. U.S. Pat. No. 5,281,849 discloses an invention, of which the present inventor was a coinventor, relating to a semiconductor package having a segmented lead frame. A U.S. Pat. No. 5,263,242 discloses and claims another aspect of the segmented lead frame technology. The above two referenced applications have addressed improvements in the field involving the substitution of a conventional lead frame, in certain applications, with a plurality of lead frame segments. However, such substitution is only a feasible alternative where it is economically practical to provide for the differences in semiconductor chip manufacturing methods attendant upon the introduction of those new inventions into the overall manufacturing process. Furthermore, the teachings of those referenced applications do not address the problem of the increased degree of precision alignment between leads which is required, particularly in plastic packages, when lead densities will be increased beyond those currently available on the market. Despite the many advances in the field made by the present inventor and others, the rapid advances in large scale integrated circuit technology are continually demanding that packaging and lead frame construction methods be improved to accommodate the ever more complex integrated circuits for which they are constructed.
One of the most significant problems facing the industry today involves the need to provide an ever increasing number of leads in a package of a generally fixed size. Presently, lead frames having 208 leads are commonly produced, but lead frames having more than 208 leads are very expensive. Using known technology, it has proven to be very difficult indeed to produce lead frames having more than 208 leads, and impossible to do so with acceptable reliability and economy.
The two primary methods for producing lead frames are either to stamp the lead frames from a metal ribbon or to use a chemical etching process to etch the lead frames. For large quantities, the chemical etching process is by far the more expensive alternative (given that the marginal cost for lead frames produced by the chemical etching process is much greater, while the fixed cost for tooling dies for the stamping process is quite large). Therefore, until recently the chemical etching method was considered to be a viable alternative only when small quantities of a particular lead frame configuration were required. Now, however, in spite of the much greater marginal costs, etched lead frames are being used exclusively for lead frames having a lead count greater than 208 leads because the stamping of such lead frames has proven to be prohibitively expensive using presently available technology. Presently available high density lead frame stamping methods require a die costing as much as, or even more than, seven hundred thousand dollars. Furthermore, such dies provide very little flexibility with, generally, only three die pad configuration options available at an additional cost of about $80,000 each. Also, as previously mentioned, the life of a 208 lead die is only about eight million units, as compared to an expected one hundred fifty million unit life for a conventional 24 lead tool.
To the inventor's knowledge, no economical and practical method for reliably producing lead frames having a lead count greater than 208 leads is known in the prior art. All lead frame construction methods for such very high lead count lead frames have required the extremely costly process of etching the entire lead frame.